A sheet-conveying apparatus of the type using a conveying table is known, for example, from DE-PS 713 529. In the apparatus shown therein, endless suction tapes circulate in the direction of the sheet travel and are guided over stationary suction containers provided with suction slits. Sheets are thus sucked to the circulating suction tapes, and are conveyed thereby. Further, conveying apparatuses of this type are known, for example, from DE 3 331 662 C2 and DE 3 838 078 A1.
DE-AS 1 033 225 discloses a sheet-conveying apparatus having endless suction tapes which are guided around the conveying table over narrow suction ducts. This apparatus holds the sheets for conveyance to the printing machine by means of subatmospheric pressure. In addition to the suction openings of the suction chambers in the table, the table further includes blowing openings. These blowing openings are provided in order to avoid the rubbing, in particular of the trailing ends, of the printed sheets on the conveying table and the resulting damage during feeding via the table. The blowing air is intended to cause the individually sucked-on sheet to rise slightly and float on an air cushion. By means of this solution, the attempt is made to reduce the static friction between sheets and conveying table. In the case of printing substrates transported in underlap-imbricated form, however, this may have the undesirable result of causing the sheets to lift off from the transporting tapes since each sheet is guided on the suction tapes only in the front region, the rear region being overlapped over the next sheet.
DE 4 012 948 C2 discloses a conveying table which has perforated suction tapes circulating over suction openings and which exhibits, in parallel along the borders of each suction tape, openings which are connected to the surroundings separately from the suction space.
The potential disadvantage with these solutions in the context of underlap-imbricated feeding is that they are directed largely to correcting for the adhesion between sheets and the conveying table. However, adherence between imbricated printing substrates may cause significant problems in cases where potential printing substrates which adhere strongly to one another are used. Such adherence may arise a result of adhesion and/or static charge and are particularly pronounced when films are used as the printing substrates. Adherence between underlap-imbricated substrates leads to the problem that--when the preceding printing substrate runs into the printing unit--it entrains the following printing substrate in the imbricated stream. This entrainment may cause a variety of problems. For example, the following printing substrate may "shoot", as a multiple sheet and/or mis-fed sheet, beyond the front lays and/or top lays. Alternatively, the following substrate may form waves and buckles when it is taken over by the pre-gripper. Further still, the following substrate may run into the printing zone. Any of these may result in deformation and jamming.